<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<style>

.box{
    height: 100px; 
    background-color: #f00;
    color: #fff;
     line-height: 100px;
}

</style>
<body>
    <div class="box">我是div</div>
</body>
</html>